The Samsung Galaxy S21 has an IP68 rating. What this means is that in terms of water-resistance, the phone is capable of remaining submerged underwater up to 1.5m deep and for a duration of 30 minutes. Presumably anything deeper or longer could result in the phone no longer working.
However, a bizarre test conducted by YouTuber Photo Owl Time Lapse has found that the IP68 rating might actually be an understatement. According to the YouTuber, they decided to test the phone’s water-resistant capabilities by dunking the phone into an aquarium from the first day they got it.
There’s even a live stream (see video above) which shows the phone underwater along with a timer, which apparently is now running for the 11th day in a row (or at least that’s the claim). According to the YouTube description, it seems that the phone did encounter some issues along the way, and apart from speakers that are reportedly damaged as a result of it being submerged for so long, the phone appears to be operational.
That being said, it is possible that because the phone isn’t submerged particularly deep (which means less pressure is being exerted on it) and the fact that the water is relatively still could have contributed to it lasting as long as it did, but it’s still a fun and interesting experiment all the same.